In automotive manufacturing, polymeric composites are being used increasingly due to their favorable characteristics, such as being lightweight, highly-conformable or shapeable, strong, and durable. Some composites are further colorable and can be finished to have most any desired texture.
The increased use in automobiles includes, for instance, in instrument and door panels, lamps, air ducts, steering wheels, upholstery, truck beds or other vehicle storage compartments, upholstery, external parts, and even engine components. Regarding engine components, and other under-the-hood (or, UTH) applications, for instance, polymers are configured, and are being developed continuously, that can withstand a hot and/or chemically aggressive environment. Regarding external parts, such as fenders, polymers are being developed that have high heat, environmental, and chemical resistance over longer periods of time. And many other potential usages in automotive applications are being considered continuously.
With the increased use of polymers, polymer composites, and other low-mass materials, compression molding and post-mold joining techniques—e.g., ultrasonic welding—are also being used more commonly.
Some conventional ultrasonic welding techniques are open-loop controlled. Upon positioning of an ultrasonic energy applicator—e.g., ultrasonic tip, referred to as a sonotrode or horn—at a point of welding, the applicator is controlled, according to an open-loop program, to descend onto the part, transmit ultrasonic vibrations following contact, and continued to descend while transmitting the vibrations. Applicator kinematics, e.g., motion, is thus controlled in the same manner, according to the program, in each iteration of welding.
For some techniques, welding systems employ a sort of closed-loop feedback control to ensure that the horn vibration is maintained at a certain, constant resonance frequency—e.g., 20 kHz.
Due to variations in welding conditions, with parameters for each iteration being kept constant (i.e., constant horn vibration frequency, amplitude of vibration, and downward force on the workpiece), the same welding function does not yield the same results in each instance. Variations include, primarily, those related to the workpieces being welded together—e.g., workpiece material, workpiece size and shape, workpiece surface roughness, workpiece surface cleanliness, and workpiece positioning and securing (e.g., clamping). Environmental conditions, and condition, including cleanliness, of the energy applicator can also be factors.
Regarding component material, one or both workpieces may contain unintended contaminants. Or workpieces can include more or less basic material than desired, such as more or less carbon-fiber than specified. The contaminants, or surplus/deficiency in make-up materials, affect workpiece reaction to welding energy. The workpieces may melt slower, or more quickly, than usual, for instance.
Also regarding workpiece material, one or both workpieces might contain defects, such as unwanted porosity. Or there might be local differences in mechanical properties of workpiece material, such as due to changes in volume fraction or orientation of fibers (e.g., carbon fibers or glass fibers) in the case of fibrous composites—a fiber-reinforced polymer (FRP) composite.
Regarding workpiece size and shape, it has been found, for example, that a relatively-slight change in volume (e.g., 2%), or difference in a local thickness or contouring, can have a relatively large affect on material melting properties.
Regarding workpiece positioning and clamping, one or both workpieces sometimes becomes malpositioned originally or due to improper clamping. Even a slight malpositioning can affect welding. Less than optimal contact between the weld energy applicator (e.g., horn) and a proximate workpiece, for instance, can lower welding effectiveness significantly in a conventional open-loop system.
Surface-related characteristics—e.g., cleanliness, roughness (rougher or smoother than specified (e.g., in a specification)), and coatings (e.g., too little or too much of a coating)—also affect the efficiency with which the ultrasonic vibrations are transferred to and through the workpieces.
Because conventional, open-loop, techniques do not accommodate variations affecting the weld process—such as contaminants in the workpiece or an unclean workpiece surface, overwelding and underwelding is common in use of those techniques.
Overwelded parts may be undesirable cosmetically, for example. Over welded joints may be weaker, e.g., due to excessive heating/melting of the material, resulting in weaker mechanical properties in and around the joint.
And overwelding may require more energy and time than desired. In a contemplated scenario, for instance, in which a welding horn is lowered until the horn lowers to a pre-set depth of the workpiece, more energy and time will be expended if the workpieces are melting slower than expected.
Underwelded parts, of course, produce weaker joints due to an undesirably low amount of bonding between the workpieces.
Underwelded and overwelded parts may need to be repaired, recycled, or scrapped.
The present technology addresses these and other shortcomings of prior welding techniques.